Display device having a density of second inorganic layer thickness direction changes in the thickness direction

ABSTRACT

A display device includes a light-emitting element layer that emits light with a luminance controlled for each of a plurality of unit pixels constituting an image, and a sealing layer provided on the light-emitting element layer and including a plurality of layers. The plurality of layers of the sealing layer includes at least an inorganic layer provided on the light-emitting element layer, an organic layer provided on the inorganic layer, and an inorganic layer that is an uppermost layer. A density of the inorganic layer that is the uppermost layer in a thickness direction changes in the thickness direction.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S.application Ser. No. 15/363,314, filed on Nov. 29, 2016, which claimspriority from Japanese application JP2016-030160 filed on Feb. 19, 2016,the content of which is hereby incorporated by reference into thisapplication.

BACKGROUND

1. Field

The present disclosure relates to a display device and a method formanufacturing a display device.

2. Description of the Related Art

A display device including a light-emitting element layer that emitslight with a luminance controlled for each of a plurality of unit pixelsconstituting an image, and a sealing layer covering the light-emittingelement layer has been conventionally known. The sealing layer isprovided for preventing moisture from entering the inside of the devicefrom the outside and reaching the light-emitting element layer. As thesealing layer, one having a three-layer structure including an inorganiclayer, an organic layer (resin layer) provided on the inorganic layer,and an inorganic layer provided on the organic layer is known asdisclosed in, for example, JP 2004-079291 A.

In recent years, the display device is required to be flexible, andensuring the resistance of the display device to bending becomes aproblem. When the sealing layer including the inorganic layer made ofsilicon nitride or the like is used as disclosed in JP 2004-079291 A,the inorganic layer may be broken at the time of bending the displaydevice. Therefore, it is conceivable to reduce the thickness of theinorganic layer for ensuring the resistance to bending. However, whenthe thickness of the inorganic layer included in the sealing layer isreduced, the sealing layer may fail to play its essential role ofpreventing the entry of moisture into the inside of the device.

SUMMARY

It is an object of the disclosure to provide a display device thatprevents the entry of moisture into the inside of the device whileensuring resistance to bending, and a method for manufacturing thedisplay device.

A display device according to an aspect of the disclosure includes: alight-emitting element layer; and a sealing layer on the light-emittingelement layer and including a plurality of layers, wherein the pluralityof layers includes at least a first inorganic layer on thelight-emitting element layer, an organic layer on the first inorganiclayer, and a second inorganic layer on the organic layer, and a densityof the second inorganic layer in a thickness direction changes in thethickness direction.

A method for manufacturing a display device according to another aspectof the disclosure includes the steps of: preparing a substrate; forminga light-emitting element layer on the substrate; and forming, on thelight-emitting element layer, a sealing layer including a plurality oflayers, wherein the step of forming the sealing layer including theplurality of layers includes the steps of forming a first inorganiclayer on the light-emitting element layer, forming an organic layer onthe first inorganic layer, and forming, at an uppermost layer of theplurality of layers, a second inorganic layer whose density in athickness direction changes in the thickness direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external appearance perspective view of a display deviceaccording to each of first to fourth embodiments.

FIG. 2 is a schematic cross-sectional view schematically showing across-section of the display device according to the first embodiment.

FIG. 3 is a circuit diagram showing a circuit formed for each of unitpixels.

FIG. 4 is a flowchart illustrating a method for manufacturing thedisplay device according to the first embodiment.

FIG. 5 is a schematic cross-sectional view schematically showing across-section of a display device according to a second embodiment.

FIG. 6 is a schematic cross-sectional view schematically showing across-section of a display device according to a third embodiment.

FIG. 7 is a schematic cross-sectional view schematically showing across-section of a display device according to a fourth embodiment.

DETAILED DESCRIPTION

Hereinafter, embodiments of the disclosure will be described withreference to the drawings.

In the embodiments of the disclosure, when the term “on” is simply usedto express the form in which one structure is disposed “on” anotherstructure, the term includes, unless otherwise noted, both the casewhere one structure is disposed directly on another structure so as tobe in contact therewith and the case where one structure is disposedabove another structure with still another structure therebetween.

First, with reference to FIGS. 1 and 2, an outline of the overallconfiguration of a display device according to a first embodiment willbe described. FIG. 1 is an external appearance perspective view of thedisplay device according to the first embodiment. FIG. 2 is a schematiccross-sectional view schematically showing a cross-section of thedisplay device according to the first embodiment. In the firstembodiment, a so-called organic electro-luminescence (EL) display deviceusing organic EL is described as the display device. However, thedisplay device is not limited to this, and it is sufficient that thedisplay device is a display device including a layer that emits lightwith a luminance controlled for each of a plurality of unit pixelsconstituting an image.

As shown in FIG. 1, the display device 100 includes a thin filmtransistor (TFT) substrate 10 including thin film transistors, and acounter substrate 20. As shown in FIG. 2, the counter substrate 20 isprovided so as to face the TFT substrate 10 with a filling material 30therebetween. Moreover, the display device 100 includes a display area Mwhere an image is displayed, and a picture-frame area N around thedisplay area M. A plurality of unit pixels P are provided in the displayarea M. Only one unit pixel P is illustrated in FIG. 1; actually,however, the plurality of unit pixels P are disposed in a matrix in thedisplay area M.

As shown in FIG. 2, the TFT substrate 10 includes a substrate 11, alight-emitting element layer 12 provided on the substrate 11, and asealing layer 13 provided on the light-emitting element layer 12 andincluding a plurality of layers. Hereinafter, the details of the layersand substrate included in the TFT substrate 10 will be described.

The substrate 11 includes at least a circuit layer including a wiringline. The details of the wiring line of the circuit layer will bedescribed later. In terms of the flexibility of the display device, thesubstrate 11 is preferably made of polyimide or the like havingflexibility. However, the substrate 11 is not limited to that and may bea glass substrate or the like.

The light-emitting element layer 12 is a layer that emits light with aluminance controlled for each of the plurality of unit pixels Pconstituting an image. The light-emitting element layer 12 is providedat least in the display area M and includes an organic EL layer 12 a, alower electrode 12 b provided below the organic EL layer 12 a, and anupper electrode 12 c provided on the organic EL layer 12 a. The detailsof the organic EL layer 12 a are not illustrated, but the organic ELlayer 12 a includes a charge transport layer, a charge injection layer,and a light-emitting layer.

An area of the organic EL layer 12 a that is in contact with the lowerelectrode 12 b corresponds to each of the unit pixels P, and lightemission is performed in this area. The unit pixels P are defined by abank layer 14. An area where the organic EL layer 12 a and the lowerelectrode 12 b are spaced apart from each other by the bank layer 14 isan area where light emission is not performed. The upper electrode 12 cis disposed on the organic EL layer 12 a over the plurality of unitpixels P. In the first embodiment, the lower electrode 12 b is an anodewhile the upper electrode 12 c is a cathode; however, the lowerelectrode 12 b and the upper electrode 12 c are not limited to them andmay be reversed in polarity. The upper electrode 12 c through whichlight from the organic EL layer 12 a passes is preferably formed as atransmissive electrode using a transparent conductive material or thelike. As the transparent conductive material, for example, indium tinoxide (ITO), indium zinc oxide (IZO), or the like is preferably used.Moreover, the upper electrode 12 c may be formed as a thin film thatallows light to pass therethrough using aluminum (Al), silver (Ag), oran alloy of silver and magnesium (Mg), or may be formed as a stackedfilm of these metal thin films and a transparent conductive material.

In the first embodiment, a separately coloring system in which theorganic EL layer 12 a is separately colored so as to emit lightscorresponding to the respective colors of the unit pixels P may beemployed, or a color filter system in which all of the unit pixels emitlights of the same color (e.g., white) and only light at a predeterminedwavelength is allowed to pass in each of the unit pixels P through acolor filter provided in the counter substrate 20 may be employed.

The sealing layer 13 is provided for preventing external moisture fromentering the inside of the display device 100 and reaching the organicEL layer 12 a. In the first embodiment as shown in FIG. 2, the sealinglayer 13 is configured by stacking an inorganic layer 13 a covering thelight-emitting element layer 12, an organic layer 13 b provided on theinorganic layer 13 a, and an inorganic layer 13 c provided on theorganic layer 13 b. In the first embodiment, the inorganic layer 13 c isthe uppermost layer of the sealing layer 13. The inorganic layers 13 aand 13 c are made of silicon nitride (SiN), but are not limited to thisas long as the inorganic layers 13 a and 13 c are made of an inorganicmaterial having high moisture resistance. For example, the inorganiclayers 13 a and 13 c may be made of silicon oxide or the like. Moreover,the organic layer 13 b is made of acrylic resin, but is not limited tothis. The organic layer 13 b may be made of epoxy resin or the like.

Further, with reference to FIGS. 2 and 3, the principle of lightemission of the light-emitting element layer will be described. FIG. 3is a circuit diagram showing a circuit formed for each of the unitpixels P. The wiring line of the circuit layer included in the substrate11 includes a scanning line Lg, a video signal line Ld orthogonal to thescanning line Lg, and a power supply line Ls orthogonal to the scanningline Lg as shown in FIG. 3. Moreover, a pixel control circuit Sc isprovided for each of the unit pixels P in the circuit layer. The pixelcontrol circuit Sc is connected to the lower electrode 12 b through acontact hole (not shown). The pixel control circuit Sc includes thinfilm transistors and a capacitor, and controls the supply of an electriccurrent to an organic light-emitting diode Od provided for each of theunit pixels P. The organic light-emitting diode Od is composed of theorganic EL layer 12 a, the lower electrode 12 b, and the upper electrode12 c, which have been described above with reference to FIG. 2.

As shown in FIG. 3, the pixel control circuit Sc includes a driver TFT11 a, a storage capacitor 11 b, and a switching TFT 11 c. The gate ofthe switching TFT 11 c is connected to the scanning line Lg, and thedrain of the switching TFT 11 c is connected to the video signal lineLd. The source of the switching TFT 11 c is connected to the storagecapacitor 11 b and the gate of the driver TFT 11 a. The drain of thedriver TFT 11 a is connected to the power supply line Ls, and theorganic light-emitting diode Od is connected to the source of the driverTFT 11 a. In response to application of a gate voltage to the scanningline Lg, the switching TFT 11 c is brought into the ON state. At thistime, when a video signal is supplied from the video signal line Ld,charge is accumulated in the storage capacitor 11 b. In response to theaccumulation of charge in the storage capacitor 11 b, the driver TFT 11a is brought into the ON state to allow an electric current to flow fromthe power supply line Ls into the organic light-emitting diode Od, sothat the organic light-emitting diode Od emits light.

It is sufficient that the pixel control circuit Sc is a circuit forcontrolling the supply of an electric current to the organiclight-emitting diode Od, and the pixel control circuit Sc is not limitedto that shown in FIG. 3. For example, the pixel control circuit Sc mayfurther include, in addition to the storage capacitor 11 b, an auxiliarycapacitance for increasing capacitance, and the polarities of thetransistors constituting the circuit are not limited to those shown inFIG. 3.

Next, the sealing layer 13 will be described further in detail. Inrecent years, while the display device is required to be flexible, theresistance of the inorganic layer included in the sealing layer 13 tobending becomes a problem. The problem is remarkable especially in theinorganic layer 13 c provided at the uppermost layer of the sealinglayer 13 which is likely to be subjected to stress. It is conceivable toreduce the thickness of the inorganic layer 13 c for ensuring theresistance to bending. In that case, however, the sealing layer 13 failsto play its essential role of preventing the entry of external moisture.

In the first embodiment, therefore, an area having a density lower thanthat of the other area is partially provided in the inorganic layer 13c. Specifically, the inorganic layer 13 c is configured to include adense film C, a sparse film B provided below the dense film C and havinga density lower than that of the dense film C, and a dense film Aprovided below the sparse film B and having a density higher than thatof the sparse film B. In the first embodiment, the density of the densefilm A and the density of the dense film C are equal to each other;however, it is sufficient that at least the densities thereof are highcompared with the sparse film B. Moreover, in the first embodiment, theinorganic layer 13 c that is composed of three films separated into thedense films and the sparse film is shown; however, the films havingdifferent densities do not need to be provided to be distinctlyseparated, and the inorganic layer 13 c may have a configuration inwhich the density changes continuously or stepwise in the thicknessdirection.

Here, the sparse film B is formed using raw materials similar to thoseof the dense films A and C, but the ratio of the raw materials to beused is different. Specifically, the sparse film B contains a largeramount of hydrogen than the dense films A and C and has correspondinglyless SiN bonds, thereby resulting in low density. The details of adeposition method of the sparse film B and the dense films A and C willbe described later.

The sparse film B of the inorganic layer 13 c, which has a density lowerthan that of the other area, is softer than the other area and has highresistance to bending. Moreover, the sparse film B provided in contactwith the dense films A and C also plays the role of relieving bendingstress applied to the dense films A and C, and thus improves theresistance of the dense films A and C to bending. Therefore, theinorganic layer 13 c as a whole has high resistance to bending, comparedwith the case where the sparse film B is not included. On the otherhand, the inorganic layer 13 c includes the dense film A and the densefilm C each having a density higher than that of the sparse film B andtherefore easily prevents the entry of external moisture compared withthe case where the density of the entire layer is lowered (the casewhere the density of the entire layer is made equal to that of thesparse film B). Since the dense film A and the dense film C areincluded, the entry of moisture can be prevented without increasing theentire thickness of the inorganic layer 13 c; therefore, the entirethickness of the display device 100 can be reduced, and thus thedownsizing of the device can also be realized. The sealing layer 13including the inorganic layer 13 c does not need to be provided on theentire surface of the display device 100, but may be provided in thedisplay area M so as to cover at least the light-emitting element layer12.

In the display device 100 according to the first embodiment as has beendescribed above, the entry of moisture into the inside of the device canbe prevented while ensuring the resistance to bending. Specifically, theresistance to bending can be ensured because the inorganic layer 13 cincludes the sparse film B while the entry of moisture into the insideof the device can be prevented because the inorganic layer 13 c includesthe dense films A and C. As a result, the organic EL layer 12 a isprevented from deteriorating due to the influence of moisture, and thusa reduction in the lifetime of the device can be reduced.

In the above description, the sparse film B has been described as beinga film having a low density compared with the dense film A and the densefilm C. In another perspective, however, the sparse film B may bedescribed as being a film having a low refractive index compared withthe dense film A and the dense film C.

Further, with reference to FIG. 4, a method for manufacturing thedisplay device according to the first embodiment will be described. FIG.4 is a flowchart illustrating the method for manufacturing the displaydevice according to the first embodiment.

First, the substrate 11 including the circuit layer is prepared (StepST1). Next, the bank layer 14 and the light-emitting element layer 12are deposited on the substrate 11 (Step ST2). Further, the inorganiclayer 13 a made of silicon nitride is deposited on the light-emittingelement layer 12 using a material containing silicon, ammonia gas, andnitrogen gas as components by a chemical vapor deposition method(hereinafter referred to as “CVD method”) (Step ST3). As the CVD method,a plasma CVD method in which source gas is converted into plasma tocause chemical reaction may be employed. In this step, nitrogen gas isused for adjusting the amount of pressure, and silicon nitride isgenerated by reaction of silicon and ammonia gas. The inorganic layer 13a is formed in a shape conforming to the shape of the light-emittingelement layer 12. Further, the organic layer 13 b made of acrylic resinis deposited on the inorganic layer 13 a (Step ST4). The surface of theorganic layer 13 b made of a resin material has a flat shape. Next, theinorganic layer 13 c made of silicon nitride, which is the uppermostlayer of the sealing layer 13, is deposited on the organic layer 13 bhaving the flat surface.

Here, the details of the deposition method of the inorganic layer 13 cwill be described. The inorganic layer 13 c is formed using a materialcontaining silicon, ammonia gas, and nitrogen gas as components by theCVD method, similarly to the inorganic layer 13 a. First, the dense filmA is deposited on the organic layer 13 b by the CVD method (Step ST5).The deposition of the dense film A is performed using the raw materialsat the same ratio as that of the inorganic layer 13 a by a step similarthereto. Then, the sparse film B is deposited on the dense film A by theCVD method (Step ST6). On this occasion, the ratio of hydrogen gas isincreased by increasing the ratio of ammonia gas for making the densityof the sparse film B lowering than that of the dense film A. Since thehydrogen gas remains inside the layer, the number of SiN bonds insidethe inorganic layer 13 c decreases as the ratio of hydrogen gasincreases. As a result, the density of the inorganic layer 13 c islowered. With this configuration, the sparse film B having a densitylower than that of the dense film A is deposited. Further, the densefilm C is deposited on the sparse film B using the raw materials at thesame ratio as that of the dense film A by a step similar thereto (StepST7). Through the steps described above, the manufacture of the TFTsubstrate 10 is completed. As has been described above, the depositionof the dense films A and C and the deposition of the sparse film B areperformed using the same raw materials, but the ratio of the rawmaterials is varied to provide a difference in density.

The method for forming the sparse film B having a low density is notlimited to that of increasing the ratio of ammonia gas, but may beperformed by adding a step of separately adding hydrogen gas duringdeposition. That is, other methods may be used as long as the ratio ofhydrogen gas in the inorganic layer 13 c can be increased. The substanceto be added is not limited to hydrogen gas, but other substances may beused as long as the substance remains inside the inorganic layer 13 cand is less likely to affect the organic EL layer 12 a. Moreover, thedeposition of the inorganic layers 13 a and 13 c is not limited to theCVD method, but other methods such as a sputtering method or an atomiclayer deposition (ALD) method may be used.

After the completion of Step ST7, the counter substrate 20 is providedso as to face the TFT substrate 10 with the filling material 30therebetween. Through the steps described above, the manufacture of thedisplay device 100 according to the first embodiment is completed.

Next, with reference to FIG. 5, a display device 200 according to asecond embodiment will be described. The display device 200 according tothe second embodiment has a configuration similar to that of the displaydevice 100, except that the stacked structure of the inorganic layer 13a is different. In the display device 200, the stacked structure of theinorganic layer 13 a is similar to the stacked structure of theinorganic layer 13 c. The external appearance of the display device 200is similar to that of the display device 100 described with reference toFIG. 1, and the principle of light emission of the display device 200 isalso similar to that of the display device 100. The same applies todisplay devices 300, 400, and 500 described later. Only theconfiguration different from that of the display device 100 is describedbelow, and a description of the common configurations is omitted.

In the second embodiment as shown in FIG. 5, the inorganic layer 13 a isconfigured by stacking a dense film D provided on the light-emittingelement layer 12, a sparse film E provided on the dense film D andhaving a density lower than that of the dense film D, and a dense film Fprovided on the sparse film E and having a density higher than that ofthe sparse film E. The sparse film E contains a larger amount ofhydrogen gas than the dense films D and F. The dense films D and F aredeposited using the raw materials at the same ratio as that of the densefilms A and C by a step similar thereto; while the sparse film E isdeposited using the raw materials at the same ratio as that of thesparse film B by a step similar thereto. The inorganic layer 13 a isprovided on the light-emitting element layer 12 and therefore has ashape conforming to the shape of the upper electrode 12 c.

In the second embodiment as has been described above, the area having adensity lower than that of the other area is provided, not only in theinorganic layer 13 c which is the uppermost layer of the sealing layer13, but also in the inorganic layer 13 a. Therefore, the resistance tobending can be further increased. In the second embodiment, theinorganic layer 13 a composed of three films separated into the densefilms and the sparse film is shown; however, the films having differentdensities do not need to be provided to be distinctly separated, and theinorganic layer 13 a may have a configuration in which the densitychanges continuously or stepwise in the thickness direction.

Next, with reference to FIG. 6, the display device 300 according to athird embodiment will be described. The display device 300 according tothe third embodiment has a configuration similar to that of the displaydevice 100, except that the stacked structure of the inorganic layer 13c is different. Only the configuration different from that of thedisplay device 100 is described below, and a description of the commonconfigurations is omitted.

In the third embodiment as shown in FIG. 6, the inorganic layer 13 c,which is the uppermost layer of the sealing layer 13, includes thesparse film B provided on the organic layer 13 b and the dense film Cprovided on the sparse film B and having a density higher than that ofthe sparse film B. That is, with respect to the inorganic layer 13 c,the display device 300 employs a configuration including two films of,from above, the dense film C and the sparse film B stacked on eachother, unlike the display device 100 including three films of, fromabove, the dense film C, the sparse film B, and the dense film A stackedon one another.

The inorganic layer 13 c, which is the uppermost layer, is not limitedto that composed of two films separated into the dense film and thesparse film, but may have a configuration in which the density changescontinuously or stepwise in the thickness direction. For example, theinorganic layer 13 c may be a layer whose density gradually decreasestoward the light-emitting element layer 12 side in the thicknessdirection.

In the third embodiment, a configuration in which the film at theuppermost layer of the inorganic layer 13 c is the dense film C and thesparse film B is provided below the dense film C is employed; however, aconfiguration in which the film at the uppermost layer of the inorganiclayer 13 c is the sparse film B and the dense film C is provided belowthe sparse film B may be employed. While the uppermost layer is morelikely to be subjected to stress, the resistance to bending can beincreased by providing the sparse film B at the uppermost layer.

The configuration in which the inorganic layer 13 c is composed of twofilms of the dense film C and the sparse film B may be applied to theinorganic layer 13 a. That is, the inorganic layer 13 a may have aconfiguration including a sparse film formed on the light-emittingelement layer 12 and a dense film formed on the sparse film and having adensity higher than that of the sparse film.

Next, with reference to FIG. 7, the display device 400 according to afourth embodiment will be described. The display device 400 according tothe fourth embodiment has a configuration similar to that of the displaydevice 100, except that an organic layer and an inorganic layer arefurther provided between the organic layer 13 b and the inorganic layer13 c in the sealing layer 13. Specifically, in the fourth embodiment, alayer having, not the three-layer structure in which the inorganic layer13 a, the organic layer 13 b, and the inorganic layer 13 c are stackedin sequence from the lower layer as shown in the first embodiment, but afive-layer structure in which the inorganic layer 13 a, the organiclayer 13 b, an inorganic layer 13 d, an organic layer 13 e, and theinorganic layer 13 c are stacked in sequence from the lower layer, isemployed as the sealing layer 13. Only the configuration different fromthat of the display device 100 is described below, and a description ofthe common configurations is omitted.

The inorganic layer 13 d has a stacked structure similar to that of theinorganic layer 13 c of the display device 100. That is, the inorganiclayer 13 d includes the dense film A, the sparse film B provided on thedense film A and having a density lower than that of the dense film A,and the dense film C having a density higher than that of the sparsefilm B.

Since the stacked structure of the five-layer structure is employed inthe display device 400 as described above, it can be said that thedisplay device 400 has a configuration that easily prevents the entry ofmoisture into the inside of the device compared with the display device100 employing the three-layer structure. Moreover, since the inorganiclayer 13 d includes the sparse film B having a density lower than thatof the other area, the resistance of the entire sealing layer 13 tobending can be correspondingly ensured.

The inorganic layer 13 a shown in each of the first to fourthembodiments corresponds to a first inorganic layer of the disclosure;the organic layer 13 b corresponds to an organic layer of thedisclosure; the inorganic layer 13 c corresponds to a second inorganiclayer that is an uppermost layer of the disclosure; the dense film Ccorresponds to a first dense film of the disclosure; and the sparse filmB corresponds to a first sparse film of the disclosure. Moreover, thedense film A included in the inorganic layer 13 c shown in each of thefirst, second and fourth embodiments corresponds to a second dense filmof the disclosure. Moreover, the dense film F included in the inorganiclayer 13 a shown in the second embodiment corresponds to a third densefilm of the disclosure, and the sparse film E corresponds to a secondsparse film of the disclosure. While there have been described what areat present considered to be certain embodiments of the disclosure, itwill be understood that various modifications may be made thereto, andit is intended that the appended claims cover all such modifications asfall within the true spirit and scope of the disclosure.

1. A method for manufacturing a display device, comprising the steps of:preparing a substrate; providing a light-emitting element layer on thesubstrate; and providing, on the light-emitting element layer, a sealinglayer including a plurality of layers, wherein the step of providing thesealing layer includes the steps of providing a first inorganic layer onthe light-emitting element layer, providing an organic layer on thefirst inorganic layer, and providing, at an uppermost layer of theplurality of layers, a second inorganic layer whose density in athickness direction changes in the thickness direction.
 2. The methodfor manufacturing a display device according to claim 1, wherein thestep of providing the second inorganic layer includes the steps ofproviding a first dense film on the organic layer, providing, on thefirst dense film, a sparse film having a density lower than that of thedense film, and providing, on the sparse film, a second dense filmhaving a density higher than that of the sparse film.
 3. The method formanufacturing a display device according to claim 1, wherein the step ofproviding the sparse film includes the step of injecting gas for makingthe density lower than that of the first dense film.